The present invention relates generally to a sulfide light-emitting layer used for inorganic EL devices, and more particularly to a fluorescent thin film used for a light-emitting layer and an EL panel using the same.
In recent years, thin-film EL devices used for small- or large-format yet lightweight flat displays have been under extensive studies. A monochromatic thin-film EL display using a fluorescent thin film comprising manganese-added zinc sulfide for emitting yellowish orange light has already been put to practical use in the form of a double-insulation type structure using such thin-film insulating layers 2 and 4 as shown in FIG. 2. Referring here to FIG. 2, a lower electrode 5 is formed on a substrate 1 in a predetermined pattern, and a first insulating layer 2 is formed on the lower electrode 5. The first insulating layer 2 is provided thereon with a light-emitting layer 3 and a second insulating layer 4 in this order, and the second insulating layer 4 is provided thereon with a predetermined pattern of an upper electrode 6 in such a way as to from a matrix circuit with the lower electrode 5.
To accommodate well to personal computer displays, TV displays and other displays, color displays are absolutely needed. Thin-film EL displays using a sulfide fluorescent material thin film are excellent in reliability and resistance to environmental conditions. At present, however, they are thought of as being unsuitable for color display purposes, because the properties of an EL fluorescent material for emitting the three primary colors or red, green and blue are less than satisfactory. Candidates for a blue emitting fluorescent material are SrS:Ce where SrS is used as a matrix material and Ce as a light emission center and ZnS:Tm, candidates for a red emitting fluorescent material are ZnS:Sm and CaS:Eu, and candidates for a green emitting fluorescent material are ZnS:Tb, CaS:Ce, etc. These materials are now under continued investigations.
These fluorescent materials for emitting the three primary colors, viz., red, green and blue have problems in conjunction with light emission luminance, efficiency, color purity, etc., and so color EL panels are still on impractical levels. For blue in particular, relatively high luminance is obtained using SrS:Ce. However, such luminance is still unsatisfactory for blue applied to full-color displays, with chromaticity shifted to a green side. Thus, much improved blue emitting layers are in great demand.
To provide a solution to these problems, thiogallate or thioaluminate blue fluorescent materials such as SrGa2S4:Ce, CaGa2S4:Ce, and BaAl2S4:Eu have been developed, as set forth in JP-A's 07-122364 and 08-134440, Shingaku Giho EID98-113, pp. 19-24, and Jpn. J. Appl. Phys. Vol. 38, (1999), pp. L1291-1292. These thiogallate fluorescent materials offer no problem in connection with color purity, but have a low luminance problem. In particular, it is very difficult to obtain uniform thin films because such materials have a multiple composition. Poor crystallizability due to poor composition controllability, defects due to sulfur release, contamination with impurities, etc. appear to be leading factors for a failure in obtaining thin films of high quality, and so resulting in no luminance increase. Thioaluminate in particular has great difficulty in composition controllability.
Thioaluminate thin films are now fabricated by a process wherein a target having the same composition as that of the BaAl2S4:Eu thin film to be obtained is prepared and this target is then used to obtain a light-emitting layer by sputtering, as shown in JP-A 08-134440, and a process wherein two pellets of BaS:Eu and Al2S3 are prepared to obtain BaAl2S4:Eu by a two-source pulse electron beam evaporation technique, as described in Jpn. J. Appl. Phys. Vol. 38, (1999), pp. L1291-1292.
JP-A 07-122364 discloses a process of obtaining an SrIn2S4:Eu light-emitting layer, wherein Sr metal, In metal and EuCl3 in the form of evaporation sources are evaporated by an MBE technique in a vacuum chamber with H2S gas introduced therein to form an SrIn2S4:Eu light-emitting layer on a substrate. With this process, however, it is very difficult to control the respective sources for the metals of a matrix material (SrIn2S4) and a light emission center material (Eu), thereby gaining precise control of the amount of the light emission center. With state-of-the-art evaporation processes, for instance, it is close to impossible to control the molar ratio of Sr and In to 1:1 for a sulfurization reaction by H2S, and regulate the molar ratio of Eu and the matrix material to 99.5:0.1 while the variation in the Ce amount of 0.1 is kept within 5% or less. Referring here to an Al electrode used as an LSI electrode, the variation of thickness of the Al thin film in an evaporation process is about 5%, although its evaporation source is kept relatively stable. From this, too, it is found that much difficulty is experienced in control of the concentration of Eu to a precision of 5% or less.
For EL thin films for other colors, i.e., red and green, on the other hand, red emitting fluorescent materials ZnS:Sm and CaS:Eu, and green emitting fluorescent materials ZnS:Tb and CaS:Ce are provided in the form of targets or pellets having the respective compositions, which are then processed by sputtering or EB evaporation to obtain fluorescent thin films capable of emitting light at relatively high luminance.
To achieve full-color EL panels, fluorescent materials capable of emitting blue, green and red light in a stable fashion and at low costs and their fabrication process are needed. However, fluorescent thin films must be fabricated by separate processes depending on their type, because the chemical or physical properties of matrix materials for the fluorescent thin films and light emission center materials differ from material to material as mentioned above. For instance, with a film formation process capable of obtaining high luminance with one single material, it is impossible to increase the luminance of a fluorescent thin film of other color. Given a full-color EL panel fabrication process, a plurality of different film formation systems are thus needed. As a result, the fabrication process increases in complexity, with an increasing panel fabrication cost.
The EL spectra of the aforesaid blue, green and red EL fluorescent thin films are all broad. When they are used for a full-color EL panel, the RGB necessary for the panel must be cut out of the EL spectra of the EL fluorescent thin films using separate filters. The use of such filters does not only make the fabrication process much more complicated, but also offer the gravest problem, viz., luminance drops. Extraction of RGB using filters causes practically unacceptable losses of 10 to 50% of the luminance of the blue, green and red EL fluorescent thin films.
To provide a solution to the aforesaid problems, there is an increasing demand for red, green and blue fluorescent thin-film materials capable of emitting light at enhanced luminance yet with improved color purity as well as a fluorescent matrix material and a light emission center material which can ensure enhanced luminance using the same film formation method or system and are similar to each other in terms of chemical or physical properties.